A typical natural valve of a mammal is the aortic valve, one of the four heart valves. The aortic valve comprises three leaflets, also called cusps, attached to the aortic root that serves as a supporting element for these leaflets. Each of the three leaflets of the aortic valve has a free margin and a margin where it is attached in semilunar fashion to the aortic root. When the valve opens, the leaflets fall back into their sinuses without the potential of occluding any coronary orifice. The hingelines of adjacent leaflets meet at the level of the sinutubular junction, forming at least part of the commissures. The body of a leaflet is pliable, extendable and thin to provide the required flexibility, although its thickness is not uniform. The leaflet is slightly thicker towards its free margin. On its ventricular surface is the zone of apposition, known as the lunule, occupying the full width along the free margin and spanning approximately one-third of the depth of the leaflet. This is where the leaflet meets the adjacent leaflets during valvular closure. With the valve in closed position, the margins of the lunules meet together, separating blood in the left ventricular cavity of the heart from blood in the aorta. For a valve of this type, or a corresponding type, highest mechanical stresses during opening and closing occur at the commissures and, to a lesser extent, at the free margin of the leaflets.
Prosthetic valves are implanted in the human or animal body and may for instance be used as a passive, one direction prosthetic valve within or nearby blood vessels. They can be completely preformed and implanted as such, or formed in situ using the artificial and/or natural parts needed to form a functional prosthetic valve. A suitable prosthetic valve needs to open and close readily in response to differential pressure on either side of the valve, cause no or only little non-physiological turbulence in the blood flow, and avoid too much regurgitation. Cardiovascular products, such as heart valve prostheses, are thus subject to high requirements with respect to loading conditions, both in magnitude as in number of cycles. Typically, heart valve leaflets may undergo over a billion load cycles in their lifetime. Durability of prosthetic valves, especially of moving leaflets, is therefore an important requirement.
Any prosthetic valve should be able to resist the actual mechanical load on the commissures and leaflet free margin during valvular operation and preferably, maintain to resist such cyclical load during many years. For this, not only initial strength is an important parameter but also reducing the chances of (non-apparent) production anomalies in making the valve.
Today, valves used in valve surgery typically are bioprosthetic valves having leaflets made from biological tissue, often chemically treated bovine pericardium. This is an elastic material that performs relatively well and is able to mimic the natural valve. However, early failure is often encountered, and is believed to be associated with high stresses on the leaflet material upon continuous stretching and retracting under pulsatile load. Various methods have been proposed as alternatives for making leaflets of prosthetic valves wherein synthetic materials and alternative designs are used.
A valve prosthesis made using synthetic fibers is for example described in NL1008349. This valve comprises a supporting element carrying a number of leaflets, which have been made by winding reinforcing fibers onto a mandrel in specific directions corresponding to the occurring stresses in the leaflets. Since the fibers have to be positioned according to the maximum stress lines, this valve prosthesis is difficult to make and uses many wound layers to accommodate stresses, whereby mass is added.
Similarly, U.S. Pat. No. 6,726,715 describes a leaflet for a heart valve comprising a flexible sheet having stress-relieving fibrous elements aligned with predetermined stress lines in the leaflet during valve operation. Sheet material is typically PTFE or PVF, with high-strength/high-modulus fibers as reinforcing elements. Fibers such as carbon, aramid, or polyethylene fibers like Dyneema® UHMWPE fibers may be used.
WO2010/020660 describes a prosthetic valve made from a uniform hollow braid made from polyolefin fibers. The hollow braid is shaped to form a valve by pulling it over a mould, comprising a tubular part and a star-shaped part. By subsequently applying heat and pressure, the hollow braid takes the shape of the mould and different sections are created. Around the tubular part of the mould the braid forms into a section that corresponds to a supporting element of the valve, whereas a star shaped part of the mould provides a section that corresponds to multiple valve leaflets. Before removing the valve from the mould, the front and back sides of the valve prosthesis are edge trimmed. To prevent disruption of the trimmed edge, the edge may be heat treated to melt fuse the yarns to each other, provided with a stitching, or otherwise treated to make the edge mechanically stable.
WO 2004/032987 concerns a medical device having at least three layers of polymeric components arranged in a sandwich construction, wherein the polymeric component of the middle layer has a shorter chain length than the other polymeric components. A heart valve is mentioned as a possible application of the sandwich construction.
Heim et al. (Materials and Manufacturing Processes, 26: 1303-1309, 2011) disclose a method wherein artificial leaflets are made from woven polyester yarns by thermally shaping the woven textile on a mould into a three-cusp geometry; showing that woven polyester could be suited to form a valve prosthesis. Polyester yarn has stretching properties such that the woven textile is able to mimic the natural elastic stretching of a human valve (about 15% of elongation), due to its typical elongation at break of about 14-17%. In order to obtain a valve with good contact between leaflets in closed position and to limit stresses during working cycles, the authors teach to shape the leaflets such that there is a fairly large inherent opening in the centre of the valve, whereas under cardiac pulsatile load adequate coaptation is created over the length of the free margin of the leaflets to prevent or at least minimize regurgitation.
In US2005/0137681 a venous valve with a tubular frame and a cover is disclosed, which cover includes surfaces defining a reversibly sealable opening and thus acting as leaflets. The leaflets can have various sizes and shapes, including arcuate edges, curved surfaces, a concave structure, or include a curved support structure to efficiently close the valve and restrict retrograde fluid flow. Leaflets may be made of biologic or synthetic fluid-impermeable material, including ePTFE, PET, urethane and polyethylene.
WO2000/62714 discloses a heart valve prosthesis including a one-piece moulded body with a plurality of leaflets, made from a silicone or polyurethane. In the neutral or rest position, the leaflets' free margins converge to form a non-uniform gap between them. The leaflets have a scallop in their free margins, proving sufficient material at the center to seal against reversed fluid flow with minimum coaptation.
US2004/176658 relates to a medical support net adapted to be placed around an organ; for example a cardiac support net, which is made as a multilayered fabric by a warp knitting technique, preferably from multifilament polyester yarn.
U.S. Pat. No. 4,191,218 discloses woven fabrics for use in vascular prostheses and heart valves, which fabrics are woven from multi-filament (polyester) yarns comprising filaments of about 10 μm diameter, and which fabrics are heat shrunk to result in open interstitial space of 20-40 μm and elongation in at least one direction of at least 10%. The fabrics preferably have a woven selvedge, which forms the free margin of a heart valve leaflet.
In US2005/177227 a method of making a cardiac valve prosthesis is disclosed, wherein a textile membrane, preferably made from polyester or PTFE, is shaped to form leaflets; for example by cutting out segments and using a shaped member reproducing the geometry of a cardiac valve in closed artery position, followed by thermofixation. It is indicated that a leaflet preferably has a woven or knitted free edge to avoid raveling.
US2012/0172978 describes a prosthetic valve comprising leaflets made from an isotropic filter screen material that has uniform pores of 15-60 μm and a thickness of 10-100 μm, and which material is woven from e.g. polyester or polypropylene monofilaments. In response to a closed flow pressure the leaflets can be pushed together to engage at the outflow edge. Methods of making such valve comprise steps of forming separately leaflets from a single layer of said screen material, coupling them together along an attachment line, and optionally coupling to a sewing ring or stent. The attachment line forms a commissure, optionally in combination with connected tabs extending from the ends of the free margin of leaflets at the outflow edge. Typically leaflets are cut from the screen material in such way that the edges of a finished leaflet do not substantially have any extending fibers.
Still, there is a continuing need for an efficient method of making implantable prosthetic valves having adequate properties for replacing a natural valve, especially for prosthetic valves showing very good durability.